1. Field of the Invention
This invention relates to cable supported and cable pulled raking arms for sedimentation thickeners, more particularly, the invention is directed to thickeners of the traction-type or center drive type, with a rake support and cabling system which eliminates the need for a thickener center structure capable of resisting large thrust forces.
2. Material Information
Thickeners or settling apparatus have been used for decades for solids-liquid separation of slimes, slurries and sludges. Generally this apparatus includes a relatively large settling tank and rotary rakes supported within the tank and which are driven to move settled material to a central area of the tank bottom. Support and driving force has been previously provided by systems of cable supports and cables for torquing or moving the rakes around the tank bottom.
U.S Pat. No. 3,295,835 shows a typical arrangement in which the rakes are supported by cables attached between the outer ends of torque drive arms and to various positions along rake carrier arms and where the drive arms and rake arms are radially displaced from each other. The cables both support and torque the rake carrier arms. The vertical component of the torque force must equal the weight of the rake arm before the arm can be lifted. Particularly when the rake arms become encrusted with heavy mud or scale any difference between that vertical component and the total weight of the rake arm and scale must be carried by the rake arm itself resulting in an inner thrust at the hinge joint and the necessity for sufficient reaction structure to absorb that thrust.
U.S. Pat. No. 3,833,126 illustrates a central vertical torque-tube or column having booms extending therefrom with pulling cables extending from spaced positions on the rake arms to an outboard position on each boom. Flexible cables extending from the center shaft or cage to the spaced positions on the rake support the rake structure. This is the so-called Swing-Lift thickener in which two torque arms extend from a rotating shaft or cage having multiple torque cables extending from the outboard end of the torque arms to spaced positions on the rake arms and support cables extending from the center cage to those same positions.
U.S. Pat. No. 3,959,152 discloses a somewhat similar cabling arrangement except that the respective sets of rake-drive and rake-support cables are attached to a traction-driven rotary drive arm of girder-type construction extending from an outer tank edge to a center column structure.
Each of the above prior art constructions particularly after scale being encountered are characterized by a cable support force which results in a thrust load in the rake arm(s) which is opposite to the cable support. Due to the requirement, as expressed in the above patents, of having the rake arms hinged so as to lift when they encounter an excess load of settled material being raked, a hinge is provided at the inboard end of the rake arms. A reaction force provided by heavy structure must be provided at the hinge to absorb the heavy thrust load inwardly along the rake arms. This thrust load and required reaction force is illustrated in Applicant's FIG. 1 prior art schematic drawing where a series of support cables provide an inward cable force which in turn creates a thrust load which must be absorbed by a relatively heavy and complex center structure or cage at the hinge point. Such structure is not only expensive but due to its size and construction can become scale encrusted so that it encroaches upon the central underflow withdrawal inlets in the tank bottom which receive the raked settled material. Scale clean-up becomes a large, more frequent job due to the amount of steel and structure needed to absorb the rake arms heavy thrust loads.